The obligate intracellular parasite Toxoplasma gondii is a major opportunistic infection of AIDS patients. Toxoplasma also causes devastating disease to fetuses and other immunocompromised patients. While much work has focused on identifying and characterizing Toxoplasma proteins and pathways important for growth and virulence of this intracellular pathogen, less is known about which host cell pathways are rate- limiting for parasite growth. Identification of these host cell processes is important because if we can inhibit them or the parasite processes dependent on them from functioning then we can block parasite growth and disease. Modulation of host cell transcription is a common mechanism to make the host cell's environment permissive for pathogen growth. During the previous funding period, we demonstrated that Toxoplasma activates a host cell transcription factor named Hypoxia Inducible Factor 1 (HIF-1) and requires HIF-1 for growth. HIF-1 activation is achieved by the parasite decreasing the abundance of the PHD2 protein, which is the key negative regulator of HIF-1. Decreases in PHD2 protein is achieved by the parasite signaling though the Activin Like Kinase (ALK) receptor family. Significantly, inhibition of ALK signaling severely impairs parasite growth. Together these data indicate that ALK/HIF-1 signaling is a key host cell determinant for Toxoplasma gondii growth and represent a novel mechanism by which a microbial pathogen subverts host cell signaling and transcription to establish its replicative niche. In this proposal we will establish which ALK receptors function during Toxoplasma infection, how PHD2 protein levels are controlled in parasite- infected cells, and define the parasite processes dependent on host ALK/HIF-1 signaling. These studies are likely to provide important information regarding the interaction between Toxoplasma and its host cell. PUBLIC HEALTH RELEVANCE: It is essential to develop new drugs against Toxoplasma gondii, which is a parasite that causes significant disease in AIDS patients as well as cancer patients and fetuses. Because the parasite can only grow inside of a host cell, we focus on and discover host cell pathways that are essential for Toxoplasma to grow. The goal of this proposal is to understand how and why Toxoplasma regulates one of these host cell pathways, which are both key steps towards developing new drugs to treat patients suffering from Toxoplasma infections.